Abstract :
Ammonia was found to react with the Brönsted acid sites of H-zeolites forming NH4+ ions, coordinate to Lewis acid sites and, at higher coverage, to NH4+ ions forming protonated ammonia associations, such as, NH4+(NH3)n. Besides the Lewis acid NH4+ ions, one kind of strong Lewis acid site was detected in zeolite H-ZSM-5 and H-mordenite, while strong and weak Lewis acid sites were substantiated to exist in zeolite H-beta. The strong acid Lewis sites were assigned to extra-framework alumina (EFAl) species generated by the partial dehydroxylation of the H-zeolites. The weak Lewis acid sites of H-beta were tentatively ascribed to framework Al atoms coordinatively unsaturated due to framework defects.
